% 纯 2D 手工绘制球
\begin{tikzpicture}[>=Stealth, scale=0.8]
    \pgfmathsetmacro{\r}{1}
    \coordinate (O) at (0, 0);      % 球心

    \draw (O) circle (\r);
    \draw (-\r,0) arc [start angle=180, end angle=360, x radius=\r, y radius=\r/2];
    \draw [dashed] (\r,0) arc [start angle=0, end angle=180, x radius=\r, y radius=\r/2];

    % 这里"绘制"一小段 arc 是为了获取坐标
    \path (-\r,0) arc [start angle=180, end angle=210, x radius=\r, y radius=\r/2] coordinate (A);
    \fill[fill=black] (O) circle (1pt);
    \draw[->] (O) -- node[above]{$r$} (A);
\end{tikzpicture}
